# BS EN 00673-2011

BSI Standards Publication BS EN 673:2011 Glass in building — Determination of thermal transmittance (U value) — Calculation methodBS EN 673:2011 BRITISH STANDARD National foreword This British Standard is the UK implementation of EN 673:2011. It supersedes BS EN 673:1998, which is withdrawn. The UK participation in its preparation was entrusted to T e c h n i c a l C o m m i t t e e B / 5 2 0 / 4 , P r o p e r t i e s a n d g l a z i n g m e t h o d s . A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. © BSI 2011 ISBN 978 0 580 71226 5 ICS 81.040.20 Compliance with a British Standard cannot confer immunity from legal obligations. This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 May 2011. Amendments issued since publication Date T e x t a f f e c t e dBS EN 673:2011EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM EN 673 February 2011 ICS 81.040.20 Supersedes EN 673:1997 English Version Glass in building - Determination of thermal transmittance (U value) - Calculation method Verre dans la construction - Détermination du coefficient de transmission thermique, U - Méthode de calcul Glas im Bauwesen - Bestimmung des U-Werts (Wärmedurchgangskoeffizient) - Berechnungsverfahren This European Standard was approved by CEN on 2 January 2011. CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member. This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions. CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG Management Centre: Avenue Marnix 17, B-1000 Brussels © 2011 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 673:2011: EBS EN 673:2011 EN 673:2011 (E) 2 Contents Page Foreword 3 Introduction .4 1 Scope 4 2 Normative references 5 3 Terms and definitions .5 4 Symbols, dimensionless numbers and subscripts 5 4.1 Symbols 5 4.2 Dimensionless Numbers .6 4.3 Subscripts 6 5 Basic formulae .7 5.1 General 7 5.2 U value 7 5.3 Radiation conductance h r.8 5.4Gas conductance h g8 5.4.1 General 8 5.4.2 Vertical glazing .9 5.4.3 Horizontal and angled glazing 9 6 Basic material properties 9 6.1 Emissivity .9 6.2 Gas properties . 10 6.3 Infrared absorption of the gas . 12 7 External and internal heat transfer coefficients 12 7.1 External heat transfer coefficient h e. 12 7.2 Internal heat transfer coefficient h i. 12 7.3 Design values 13 8 Declared values: standardized boundary conditions . 13 9 Expression of the results . 14 9.1 U values . 14 9.2 Intermediate values 14 10 Test report . 14 10.1 Information included in the test report . 14 10.2 Identification of the glazing . 14 10.3 Cross section of the glazing 15 10.4 Results . 15 Annex A (normative) Iteration procedure for glazing with more than one gas space 16 Bibliography . 18 BS EN 673:2011 EN 673:2011 (E) 3 Foreword This document (EN 673:2011) has been prepared by Technical Committee CEN/TC 129 “Glass in building”, the secretariat of which is held by NBN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by August 2011, and conflicting national standards shall be withdrawn at the latest by August 2011. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document supersedes EN 673:1997. This document has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directive(s). According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. BS EN 673:2011 EN 673:2011 (E) 4 Introduction CEN/TC 129/WG9 “Light and energy transmission, thermal insulation“ prepared a working draft based on the document ISO/DIS 10292, “Thermal insulation of glazing: Calculation rules for determining the steady state U value of double or multiple glazing“, document that was prepared by ISO/TC 160, “Glass in building“. This was published in 1997 as EN 673. This edition is a revision of EN 673:1997. The main change in this edition is that the internal and external heat transfer coefficients have been amended slightly to avoid any ambiguities. The original annex on the determination of emissivity has been removed and reference is made to EN 12898. Other changes include the incorporation of amendments A1 and A2 to EN 673:1997 and general improvements to the text to aid understanding. 1 Scope This European Standard specifies a calculation method to determine the thermal transmittance of glazing with flat and parallel surfaces. This European Standard applies to uncoated glass (including glass with structured surfaces, e.g. patterned glass), coated glass and materials not transparent in the far infrared which is the case for soda lime glass products, borosilicate glass and glass ceramic. It applies also to multiple glazing comprising such glasses and/or materials. It does not apply to multiple glazing which include in the gas space sheets or foils that are far infrared transparent. The procedure specified in this European Standard determines the U value 1) (thermal transmittance) in the central area of glazing. The edge effects due to the thermal bridge through the spacer of a sealed glazing unit or through the window frame are not included. Furthermore, energy transfer due to solar radiation is not taken into account. The effects of Georgian and other bars are excluded from the scope of this European Standard. The standard for the calculation of the overall U value of windows, doors and shutters (see EN ISO 10077-1 [1]) gives normative reference to the U value calculated for the glazing components according to this standard. For the purpose of product comparison, a vertical position of the glazing is specified. In addition, U values are calculated using the same procedure for other purposes, in particular for predicting: heat loss through glazing; conduction heat gains in summer; condensation on glazing surfaces; the effect of the absorbed solar radiation in determining the solar factor (see Bibliography, [2]). Reference should be made to [3], [4] and [5] or other European Standards dealing with heat loss calculations for the application of glazing U values determined by this standard. A procedure for the determination of emissivity is given in EN 12898. 1) In some countries the symbol k has been used hitherto. BS EN 673:2011 EN 673:2011 (E) 5 The rules have been made as simple as possible consistent with accuracy. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 674, Glass in building — Determination of thermal transmittance (U value) — Guarded hot plate method EN 675, Glass in building — Determination of thermal transmittance (U value) — Heat flow meter method EN 12898, Glass in building — Determination of the emissivity 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 U value parameter of glazing which characterizes the heat transfer through the central part of the glazing, i.e. without edge effects, and states the steady-state density of heat transfer rate per temperature difference between the environmental temperatures on each side NOTE The U value is given in watts per square metre Kelvin [W/(m 2 ⋅K)]. 3.2 declared value U value obtained under standardized boundary conditions (see Clause 8) 4 Symbols, dimensionless numbers and subscripts 4.1 Symbols A c o n s t a n t - c specific heat capacity of gas J/(kg⋅K) d thickness of material layer (glass or alternative glazing material) m F v o l u m e f r a c t i o n - h - heat transfer coefficient W/(m 2 ⋅K) - also thermal conductance W/(m 2 ⋅K) M number of material layers - n exponent - N number of spaces - BS EN 673:2011 EN 673:2011 (E) 6 r thermal resistivity of glass (glazing material) m⋅K/W P gas property - s width of gas space m T absolute temperature K U thermal transmittance W/(m 2 ⋅K) ∆ Τtemperature difference K εcorrected emissivity - ε n normal emissivity (perpendicular to the surface) - ρg a s d e n s i t y k g / m 3σStefan-Boltzmann s constant 5,67 x 10 -8W / ( m 2 ⋅K 4 ) µ dynamic viscosity of gas kg/(m⋅s) λ- thermal conductivity of gas in space W/(m⋅K) ϑtemperature on the Celsius scale °C 4.2 Dimensionless Numbers Gr Grashof number - Nu N u s s e l t n u m b e r - Pr Prandtl number - 4.3 Subscripts c convection e external i internal j j thmaterial layer k k thspace g gas m mean n normal r radiation s space BS EN 673:2011 EN 673:2011 (E) 7 t total 1;2 first, second etc. 5 Basic formulae 5.1 General The method of this standard is based on a calculation according to the following principles. 5.2 U value The U value is given by: 1 U = 1 h e + 1 h t + 1 h i(1) where h eand h iare the external and internal heat transfer coefficients; h tis the total thermal conductance of the glazing. 1 h t = 1 h s 1 N ∑ + d j 1 M ∑ ⋅ r j(2) where h sis the thermal conductance of each gas space; N is the number of spaces; d jis the thickness of each material layer; r jis the thermal resistivity of each material (thermal resistivity of soda lime glass = 1,0 m⋅K/W); M is the number of material layers. h s,k= h r,k+ h g,k(3) where h s,kis the heat transfer of the k thspace; h r,k is the radiation conductance; h g,kis the U value of gas. NOTE The thermal resistivity of components other than glass (e.g. interlayers in laminated glass) may be taken into account in determining the U value. For the purpose of this standard, thermal conductivity values used for glass in building should be obtained from the table of generally accepted values in the relevant product standard (e.g. EN 572-1 for basic soda lime silicate glass). In instances where the effects are not considered significant or important, a simplified approach may be taken, i.e. ignoring the effects of components other than glass. BS EN 673:2011 EN 673:2011 (E) 8 5.3 Radiation conductance h rThe radiation conductance is given by: 3 k m, 1 k , 2 k , 1 r 1 1 1 4 T h − − + = ε ε σ (4) where σ is the Stefan-Boltzmann s constant; T m,kis the mean absolute temperature of the gas space; ε 1,k and ε 2,k are the corrected emissivities of the surfaces bounding the enclosed space between the panes at T m,k . 5.4 Gas conductance h g 5.4.1 General The gas conductance is given by: k k k , g s h λ Nu = (5) where s kis the width of the k thspace; λ k is the thermal conductivity of the k thgas; Nu is the Nusselt number. Nu = A ⋅ (Gr ⋅ Pr) n(6) where A is a constant; Gr is the Grashof number; Pr is the Prandtl number; n is an exponent. Gr = 9,8 1 s 3 ∆ T ⋅ ρ 2 T m µ 2(7) Pr = µ c λ(8) BS EN 673:2011 EN 673:2011 (E) 9 where ∆ T is the temperature difference between glass surfaces bounding the gas space; ρis the density; µ is the dynamic viscosity; c is the specific heat capacity; T m is the mean temperature. The Nusselt number is calculated from Equation (6). If Nu is less than 1, then the value unity is used for Nu in Equation (5). 5.4.2 Vertical glazing For vertical glazing: A is 0,035 n is 0,38 5.4.3 Horizontal and angled glazing For horizontal or angled glazing and upward heat flow the heat transfer by convection is enhanced. This effect shall be considered by substituting the following values of A and n in Equation (6). Horizontal spaces A = 0,16 n = 0,28 Space at 45° A = 0,10 n = 0,31 For intermediate angles linear interpolation is satisfactory; however, the linear interpolation shall be between the two nearest points. When the direction of heat flow is downward the convection shall be considered suppressed for practical cases and Nu = 1 is substituted in Equation (5). 6 Basic material properties 6.1 Emissivity The corrected emissivities εof the surfaces bounding the enclosed spaces are required to calculate the radiation conductance h rin Equation (4). For uncoated soda lime glass surfaces or for soda lime glass surfaces with coatings which have no effect on the emissivity, the corrected emissivity to be used is 0,837. NOTE 1 With reasonable confidence the same value may be used for uncoated borosilicate glass. For other coated surfaces the normal emissivity ε n shall be determined with an infrared spectrometer in accordance with EN 12898. The corrected emissivity shall be determined from the normal emissivity in accordance with EN 12898. BS EN 673:2011 EN 673:2011 (E) 10 NOTE 2 Two different definitions of emissivity should be theoretically used to describe radiation exchange between: a) glass surfaces facing each other in glazing; b) a glass surface facing a room. However, in practice numerical differences are found to be negligibly small. Thus corrected emissivity describes both types of heat exchange with a sufficient approximation. NOTE 3 For laminated glass and laminated safety glass where a low emissivity coating is in direct contact with an interlayer, the effect of the low emissivity coating is negated, in terms of U value. 6.2 Gas properties The properties of the gas filling the sp